Method for controlling fuel splits to gas turbine combustor

ABSTRACT

A method for determining a target exhaust temperature for a gas turbine including: determining a target exhaust temperature based on a compressor pressure condition; determining a temperature adjustment to the target exhaust temperature based on at least one parameter of a group of parameters consisting of specific humidity, compressor inlet pressure loss and turbine exhaust back pressure; and adjusting the target exhaust temperature by applying the temperature adjustment.

BACKGROUND OF THE INVENTION

The present invention relates generally to controllers for a combustionsystem for a gas turbine. In particular, the invention relates to acombustor control algorithm for fuel splits and nitrogen oxide/nitrogendioxide (NOx) leveling.

Industrial and power generation gas turbines have control systems(“controllers”) that monitor and control their operation. Thesecontrollers govern the combustion system of the gas turbine. Dry Low NOx(DLN) combustion systems are designed to minimize emissions of NOx fromgas turbines. The controller executes an algorithm to ensure safe andefficient operation of the DLN combustion system. Conventional DLNalgorithms receive as inputs measurements of the actual exhausttemperature of the turbine and the actual operating compressor pressureratio. DLN combustion systems typically rely on the measured turbineexhaust temperature and compressor pressure ratio to set the gas turbineoperating condition, e.g., desired turbine exhaust temperature, totalcombustor fuel flow, fuel split schedules, and inlet bleed heat flow.

Conventional scheduling algorithms for DLN combustion systems do notgenerally take into account variations in compressor inlet pressureloss, turbine back pressure, or compressor inlet humidity. Conventionalscheduling algorithms generally assume that ambient conditions, e.g.,compressor inlet humidity, compressor inlet pressure loss, and turbineback pressure remain at certain defined constant conditions or thatvariations in these conditions do not significantly affect the targetcombustor firing temperature.

Compressor inlet pressure loss and turbine back-pressure levels willvary from those used to define the DLN combustion settings. The NO_(x)emissions from the gas turbine may increase beyond prescribed limits, ifthe conventional DLN combustion system is not adjusted as environmentalconditions change. Seasonal variations in humidity or changes in turbineinlet humidity from various inlet conditioning devices, e.g.,evaporative cooler, fogging systems, can influence the operation of aDLN combustion system. As the ambient conditions change with theseasons, the settings of DLN combustion systems are often manuallyadjusted to account for ambient seasonal variations.

BRIEF SUMMARY OF THE INVENTION

The invention may be embodied as a method for scheduling a fuel splitfor a gas turbine combustor comprising: determining a target exhausttemperature that would produce the desired NOx at the reference fuelsplits (based on at least one parameter of a group of parametersconsisting of specific humidity, compressor inlet pressure loss andturbine exhaust back pressure); determining an exhaust temperature errorbased on the difference between the scheduled exhaust temperature andthe exhaust temperature that would produce the desired NOx at thereference fuel splits; converting the exhaust temperature error to aprojected NOx level error at the reference fuel splits, and convertingthe projected NOx level error to adjustments to the fuel split levels.

The invention may also be embodied as a method for scheduling a fuelsplit for a gas turbine combustor comprising: determining a targetexhaust temperature corresponding to a desired NOx at a reference fuelsplit; determining an exhaust temperature error based on a comparisonbetween a scheduled exhaust temperature and the target exhausttemperature; converting the exhaust temperature error to a projected NOxleveling fuel split adjustment; adjusting the reference fuel split usingthe projected NOx leveling fuel split adjustment, and applying theadjusted fuel split to determine fuel flow to the combustor.

The invention may be further embodied as a method for adjusting a basefuel split schedule for a gas turbine combustor comprising: determininga corrected target turbine exhaust temperature based on a compressorpressure condition; determining a first corrected temperature adjustmentto the corrected target exhaust temperature based on at least oneparameter of a group of parameters consisting of compressor inletpressure loss and turbine exhaust back pressure; determining a secondcorrected temperature adjustment to the corrected target exhausttemperature based on a nitrogen oxide (NOx) limiting requirement and thebase fuel split command; determining an adjusted corrected targetexhaust temperature based on the first corrected temperature adjustmentand the second corrected temperature adjustment; determining anuncorrected adjusted target exhaust temperature based on a temperatureof a discharge of a compressor of the gas turbine and the adjustedcorrected target exhaust temperature; determining a temperaturedifference between the uncorrected adjusted corrected target exhausttemperature and an uncorrected target exhaust temperature selected froma combustor temperature leveling algorithm, and applying the temperaturedifference to generate an adjusted base fuel split schedule.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings in conjunction with the text of thisspecification describe an embodiment of the invention.

FIG. 1 is a schematic depiction of a gas turbine having a fuel controlsystem.

FIG. 2 is a high-level block diagram of a system for selecting a desiredturbine exhaust temperature and fuel split schedule.

FIG. 3 is a diagram of an exemplary algorithm for calculating theexhaust temperature that would produce the desired NOx at the referencefuel splits.

FIG. 4 is a diagram of an exemplary base fuel split adjustment algorithmfor a NOx leveling algorithm.

DETAILED DESCRIPTION OF THE INVENTION

A gas turbine control system and method of algorithms has been developedto schedule the operation of a gas turbine such that the turbine exhausttemperature and combustor fuel splits are cooperatively scheduled. Bylinking algorithms for determining the turbine exhaust temperature andfuel splits, the gas turbine control system can simultaneously level thecombustor temperature rise (when not otherwise limited), and NOxemissions. This feature is especially useful during part-load gasturbine operation.

Part load operation of a gas turbine often involves a control systemadjusting a total fuel flow to achieve the part-load level and adjustingthe compressor inlet guide vane (IGVs) to set the gas turbine cyclematch point for the desired part-load level. Further, the controllerschedules the fuel splits for the combustor to maintain the desiredcombustion mode, e.g., part-load total fuel flow, and operate the gasturbine within established operability boundaries, such as forcombustion dynamics. Moreover, during part-load operation, the cyclematch point and combustor fuel splits greatly influence the NOxemissions. To optimize the operation of the gas turbine during part loadoperation, the controller simultaneously applies a NOx levelingalgorithm and other algorithms to control the combustor temperaturerise.

FIG. 1 depicts a gas turbine 10 having a compressor 12, combustor 14,turbine 16 drivingly coupled to the compressor, and a control system(controller) 18. An inlet duct 20 to the compressor feeds ambient airand possibly injected water to the compressor. The inlet duct may haveducts, filters, screens and sound absorbing devices that contribute to apressure loss of ambient air flowing through the inlet 20 into inletguide vanes 21 of the compressor. An exhaust duct 22 for the turbinedirects combustion gases from the outlet of the turbine through, forexample, emission control and sound absorbing devices. The exhaust duct22 may include sound adsorbing materials and emission control devicesthat apply a backpressure to the turbine. The amount of inlet pressureloss and back pressure may vary over time due to the addition ofcomponents to the ducts 20,22, and to dust and dirt clogging the inletand exhaust ducts. The turbine may drive a generator 24 that produceselectrical power. The inlet loss to the compressor and the turbineexhaust pressure loss tend to be a function of corrected flow throughthe gas turbine. Further, the amount of inlet loss and turbine backpressure may vary with the flow rate through the gas turbine.

The operation of the gas turbine may be monitored by several sensors 26detecting various conditions of the turbine, generator and ambientenvironment. For example, temperature sensors 26 may monitor ambienttemperature surrounding the gas turbine, compressor dischargetemperature, turbine exhaust gas temperature, and other temperaturemeasurements of the gas stream through the gas turbine. Pressure sensors26 may monitor ambient pressure, and static and dynamic pressure levelsat the compressor inlet and outlet, turbine exhaust, at other locationsin the gas stream through the gas turbine. Humidity sensors 26, e.g.,wet and dry bulb thermometers, measure ambient humidity in the inletduct of the compressor. The sensors 26 may also comprise flow sensors,speed sensors, flame detector sensors, valve position sensors, guidevane angle sensors, or the like that sense various parameters pertinentto the operation of gas turbine 10. As used herein, “parameters” referto items that can be used to define the operating conditions of turbine,such as temperatures, pressures, and gas flows at defined locations inthe turbine. These parameters can be used to represent a given turbineoperating condition.

A fuel control system 28 regulates the fuel flowing from a fuel supplyto the combustor 14, and the split between the fuel flowing into primaryand secondary fuel nozzles, and the fuel mixed with secondary airflowing into a combustion chamber. The fuel controller may also selectthe type of fuel for the combustor. The fuel control system 28 may be aseparate unit or may be a component of a larger controller 18. The fuelcontrol system may also generate and implement fuel split commands thatdetermine the portion of fuel flowing to primary fuel nozzles and theportion of fuel flowing to secondary fuel nozzles.

The controller 18 may be a General Electric SPEEDTRONIC™ Gas TurbineControl System, such as is described in Rowen, W. I., “SPEEDTRONIC™ MarkV Gas Turbine Control System”, GE-3658D, published by GE Industrial &Power Systems of Schenectady, N.Y. The controller 18 may be a computersystem having a processor(s) that executes programs to control theoperation of the gas turbine using sensor inputs and instructions fromhuman operators. The programs executed by the controller 18 may includescheduling algorithms for regulating fuel flow to the combustor 14. Thecommands generated by the controller cause actuators on the gas turbineto, for example, adjust valves (actuator 32) between the fuel supply andcombustors that regulate the flow, fuel splits and type of fuel flowingto the combustors; adjust inlet guide vanes 21 (actuator 30) on thecompressor, and activate other control settings on the gas turbine.

The controller 18 regulates the gas turbine based, in part, onalgorithms stored in computer memory of the controller. These algorithmsenable the controller 18 to maintain the NOx and CO emissions in theturbine exhaust to within certain predefined emission limits, and tomaintain the combustor firing temperature to within predefinedtemperature limits. The algorithms have inputs for parameter variablesfor current compressor pressure ratio, ambient specific humidity, inletpressure loss and turbine exhaust back pressure. Because of theparameters in inputs used by the algorithms, the controller 18accommodates seasonal variations in ambient temperature and humidity,and changes in the inlet pressure loss through the inlet 20 of the gasturbine and in the exhaust back pressure at the exhaust duct 22. Anadvantage of including input parameters for ambient conditions, andinlet pressure loss and exhaust back pressure is that the NO_(x), CO andturbine firing algorithms enable the controller to automaticallycompensate for seasonal variations in gas turbine operation and changesin inlet loss and in back pressure. Accordingly, the need is reduced foran operator to manually adjust a gas turbine to account for seasonalvariations in ambient conditions and for changes in the inlet pressureloss or turbine exhaust back pressure.

The combustor 14 may be a DLN combustion system. The control system 18may be programmed and modified to control the DLN combustion system. TheDLN combustion control algorithms for determining fuel splits are setforth in FIGS. 2 to 5.

The schedules and algorithms executed by the controller 18 accommodatevariations in ambient conditions (temperature, humidity, inlet pressureloss, and exhaust back pressure) that affect NOx, combustor dynamics,and firing temperature limits at part-load gas turbine operatingconditions. The control system 18 simultaneously schedules exhausttemperature and combustor fuel splits. The control system 18 appliesalgorithms for scheduling the gas turbine, e.g., setting desired turbineexhaust temperatures and combustor fuel splits, so as to satisfyperformance objectives while complying with operability boundaries ofthe gas turbine. The turbine control system 18 simultaneously determineslevel combustor temperature rise and NOx during part-load operation inorder to increase the operating margin to the combustion dynamicsboundary and thereby improve operability, reliability, and availabilityof the gas turbine.

The combustor fuel splits are scheduled by the control system 18 tomaintain the desired combustion mode while observing other operabilityboundaries, such as combustion dynamics. At a given load level, thecycle match point and the combustor fuel splits influence the resultantNOx emissions. Simultaneously leveling NOx and combustor temperaturerise during part-load operation minimizes the level of combustiondynamics and expands the operational envelope of the gas turbine withoutadversely impacting emissions compliance or parts life.

FIG. 2 is a high-level block diagram of a process 34 for selecting adesired turbine exhaust temperature target (Tx_req) and adjusted fuelsplit (PM1, PM3) 39 and 41. The process and associated algorithmsdisclosed herein are primarily directed to a NOx leveling algorithm 74(FIG. 4) that determines a fuel split adjustment. The NOx levelingalgorithm requires as an input the actual scheduled turbine exhausttemperature target (Tx_req) 38. The manner in which the desired turbineexhaust temperature target is determined may vary without departing fromthe scope or intent of the technique for applying the NOx levelingalgorithm 74 to determine adjusted fuel split input parameters (PM1,PM3) 39 and 41 for the fuel controller 28.

The overall process 34 includes a selection logic 36 that selects acombustion exhaust temperature target (Tx_req) 38 from a plurality ofproposed temperatures by applying a certain logic, such as selection ofthe lowest temperature of the input temperature targets. These proposedexhaust temperature targets include: a maximum exhaust temperature(Iso-Therm), a desired exhaust temperature (Tx_Tf) generated by a firingtemperature leveling algorithm 40, a desired exhaust temperature (Tx_Tr)generated by a combustor temperature rise leveling algorithm 42, and adesired exhaust temperature (Tx_NOx) generated by a combustortemperature rise leveling algorithm 44. The scheduled exhausttemperature 38 is compared by the controller 18 to the actual turbineexhaust temperature. The difference between the desired and actualexhaust temperatures is applied by the controller to regulate the fuelflow to the combustor or the angle of the IGVs 21 (when operatingpart-load).

The NOx leveling algorithm 44 outputs an adjusted fuel split commands(PM1, PM3) 39 and 41 to the fuel controller 28. The fuel split commandsare collectively the fuel split schedule. The fuel split commandsindicates the portion of the fuel that is to flow to the various zonesof fuel injectors, e.g., primary fuel nozzle and secondary fuelinjectors for mixing fuel with secondary air entering the combustors.The NOx leveling algorithm is one technique to reduce NOx emissions fromthe turbine and to maintain NOx emissions within emission limits.

FIG. 3 is a block diagram illustrating the NOx leveling algorithm 44that relies on a relatively simple and easily executed exhausttemperature versus compressor pressure ratio (X_(c)) relationships 46 todetermine the desired operating conditions, e.g., fuel splits andexhaust temperature. These relationships are stored in the electronicmemory of the control system 18 and may include data look-up tables,mathematical functions (such as first or second order curve functions)and other forms of electronically representing a parameter relationship.

The NOx leveling algorithm 44 receives input data regarding the currentcompressor pressure ratio (Xc or CPR), a NOx emission limit (NOx_req) 84(which is a sum of the NOx emission limit and a delta NOx tuning factor(ΔNOx_tune) that is a constant NOx adjustment determined for eachspecific gas turbine), the current specific humidity (SH) 68 of theambient air entering the compressor 12, an inlet pressure loss ΔPin 52,and a turbine back pressure delta ΔPex 56. Based on these inputparameters, the NOx algorithm 44 produces an exhaust temperature 70 thatwould produce the desired NOx level at the reference fuel splits.

The NOx leveling algorithm 44 includes a schedule 46 for applying thecompressor pressure ratio (Xc) to derive a corrected turbine exhausttemperature target 48 (T_corr) for the NOx algorithm. The schedule 46 ofcorrected exhaust temperature versus compressor pressure ratio (controlcurve) outputs a corrected turbine exhaust temperature target 48(T_corr) for defined reference parameter conditions of: NOx target, fuelsplits, humidity, inlet pressure loss, and exhaust back pressure. Thecompressor pressure ratio vs. exhaust temperature target schedule 46 maybe a graph, look-up table or function that correlates the compressorpressure ratio to a corrected exhaust temperature target 48. Theschedule 46 is determined for a gas turbine or gas turbine type or classin a conventional manner that is outside the scope of the presentinvention.

The corrected exhaust temperature 48 is adjusted to correct foroff-reference inlet pressure loss (ΔPin), off-reference exhaust backpressure (ΔPex), off-reference NOx target, and off-reference humidity.An inlet pressure loss function (f(ΔPin)) 52 is applied to determine anadjustment to the corrected exhaust temperature due to a difference (ΔPin) in actual inlet pressure loss from a base inlet pressure losslevel. The inlet pressure loss function may be an empirically derivedgraph, look-up table or function that correlates the inlet pressure lossdifference (ΔPin) to an adjustment 54 to the corrected exhausttemperature target 48. This function 52 may be derived for a particularclass, model, or type of gas turbine and/or for a particular arrangementof inlet ducts and inlet components. Alternatively, the pressure lossfunction may have input variables of the inlet pressure loss difference(ΔPin) and the compressor ratio level (CPR). The pressure loss function52 generates a temperature target adjustment 54 to be summed with thecorrected exhaust temperature target 48.

Similarly, a turbine back pressure function (ΔPex) 56 has an inputvariable of a difference (ΔPex) between the actual turbine back pressureand a base turbine back pressure level. The back pressure function 56may have CPR as a second input variable. The turbine back pressurefunction generates a temperature target adjustment 58 to be summed withthe corrected exhaust temperature 48. A further temperature targetadjustment (ΔTx_Tune) 60 is generated from an adjusted target NOx level84 (NOx_req) and is summed with the corrected exhaust temperature target48. The corrected exhaust temperature target 48 (after being adjusted toaccount for compressor inlet pressure loss, exhaust back pressure andtarget NOx level) is uncorrected by applying a correction factor 64 thatis a non-integer exponent of a ratio of the compressor dischargetemperature (TCD) and a reference TCD. The value of the non-integerexponent is empirically derived for the class or model of gas turbine.The correction factor 64 is the inverse of the correction factor used tocollapse the part-load exhaust temperature versus TCD data over the loadrange of the gas turbine in FIG. 4.

The uncorrected exhaust temperature target is further summed with atemperature target correction 66 generated by a specific humidityfunction (f(SH)) 68 that is an empirically derived function having asinputs the ambient specific humidity and, possibly, the compressor ratio(CPR). The result is the exhaust temperature target (Tx_NOx) 70 that isthe exhaust temperature that would produce the desired NOx level at thereference fuel splits under the current operational conditions.

FIG. 4 is a schematic diagram of the fuel split scheduling algorithm 74which generates the adjusted fuel splits (PM1, PM3). The adjustments(DPM1 and DPM3) to the base fuel splits (PM1_base and PM3_base) reflectNOx leveling adjustments to the fuel splits. This algorithm 74 accept asinput the exhaust temperature (Tx_NOx) 70 that would produce the desiredNOx level at the reference fuel splits, and the actual scheduled exhausttemperature (Tx_req) 38. A delta exhaust temperature difference (Tx_err)88 is the difference between the temperature level 70 that would levelNOx to the target and the temperature 38 that is actually scheduled. Thetemperature difference (Tx_err) 70 is limited to a positive value by aminimum select logic 90 that selects the larger of a positive differenceTx_err or zero. The positive temperature difference Tx_err is correctedby applying the correction factor 64 that is a non-integer exponent of aratio of the compressor discharge temperature (TCD) and a reference TCD.In addition the corrected temperature difference is summed with thecorrected NOx temperature correction (ΔTx_Tune) 60 generated by the NOxtarget adjustment. The summed corrected temperature difference 92 isconverted to a pseudo NOx target 94 via a non-linear, empiricallyderived curve 86, which is the same curve used to convert the NOx_req 84factor to the NOx temperature difference 60.

The pseudo NOx target 94 is inverted and multiplied with the requestedNOx level (NOx_req) 84 to yield a Q adjust (Q_adj) ratio (Q_adj) 96 ofpseudo NOX target/NOx_req. The adjusted Q factor (Q_adj) 96 is convertedto the delta-PM1 request command (DPM1) using the non-linear schedule 76that relates the required delta-PM1 fuel split adjustment (command DPM1)to the Q factor 96. This schedule 76 is developed based on the effect offuel splits on NOx emissions. The schedule 76 projects the NOx levelingerror that would result in the base fuel splits would be applied to thecombustor. Built into this schedule 76 is a prescribed relationshipbetween PM1 and PM3. The delta-PM1 adjustment command (DPM1) is used togenerate a delta-PM3 request command (DPM3) using the non-linearschedule 98 that reflects the relationship between DPM3 and DPM1. Thefuel split requests (PM1, PM3) are generated as the sum of thedelta-fuel splits (DPM1 and DPM3) and the base-load fuel split levels(PM1_base, PM3_base), respectively. The fuel split request commands(PM1, PM3) are used by the fuel control to govern the portion of fuelflowing to the various fuel nozzles.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for scheduling a fuel split for a gas turbine combustorcomprising: a. determining a target exhaust temperature corresponding toa desired nitrogen oxide (NOx) at a reference fuel split; b. determiningan exhaust temperature error based on a comparison between a scheduledexhaust temperature and the target exhaust temperature; c. convertingthe exhaust temperature error to a projected NOx leveling fuel splitadjustment; d. adjusting the reference fuel split using the projectedNOx leveling fuel split adjustment, and e. applying the adjusted fuelsplit to determine fuel flow to the combustor.
 2. A method as in claim 1wherein the target exhaust temperature is determined based on at leastone parameter of a group of parameters consisting of specific humidity,compressor inlet pressure loss and turbine exhaust back pressure.
 3. Amethod as in claim 1 wherein the fuel split level is a plurality of fuelsplit levels each indicating a portion of fuel flow to one of aplurality of zones of fuel nozzles in the combustor.
 4. A method as inclaim 1 wherein steps (b) through (d) are performed in connection with anitrogen oxide (NOx) leveling algorithm.
 5. A method as in claim 1wherein the conversion of the exhaust temperature error to the projectedNOx leveling adjustment further comprises: determining a projected NOxlevel from the exhaust temperature error; determining an adjusted Qfactor as a ratio of a NOx level baseline request and the projected NOxlevel, and applying the adjusted Q factor to determine the projected NOxlevel adjustment.
 6. A method as in claim 5 further comprisingdetermining the adjusted NOx level from the exhaust temperature error.7. A method as in claim 6 wherein the determination of the adjusted NOxlevel from the exhaust temperature error further comprises correctingthe exhaust temperature error account for a condition of a compressor inthe gas turbine, and applying the corrected exhaust temperature error toan empirically derived curve relating corrected exhaust temperatureerror to the adjusted NOx level.
 8. A method as in claim 7 wherein thecondition of the compressor is a temperate of compressed air dischargedfrom the compressor.
 9. A method as in claim 1 wherein the comparisonused to determine the exhaust temperature error is a difference betweena scheduled exhaust temperature and the target exhaust temperature, andsaid difference is the exhaust temperature error.
 10. A method foradjusting a base fuel split schedule for a gas turbine combustorcomprising: a. determining a corrected target turbine exhausttemperature based on a compressor pressure condition; b. determining afirst corrected temperature adjustment to the corrected target exhausttemperature based on at least one parameter of a group of parametersconsisting of compressor inlet pressure loss and turbine exhaust backpressure; c. determining a second corrected temperature adjustment tothe corrected target exhaust temperature based on a nitrogen oxide (NOx)limiting requirement and the base fuel split command; d. determining anadjusted corrected target exhaust temperature based on the firstcorrected temperature adjustment and the second corrected temperatureadjustment; e. determining an uncorrected adjusted target exhausttemperature based on a temperature of a discharge of a compressor of thegas turbine and the adjusted corrected target exhaust temperature; f.determining a temperature difference between the uncorrected adjustedcorrected target exhaust temperature and an uncorrected target exhausttemperature selected from a combustor temperature leveling algorithm,and g. applying the temperature difference to generate an adjusted fuelsplit schedule.
 11. A method as in claim 10 wherein the uncorrectedadjusted target exhaust temperature is further based on an uncorrectedtemperature adjustment determined from ambient specific humidity.
 12. Amethod as in claim 10 wherein the difference between the uncorrectedadjusted corrected target exhaust temperature and an uncorrected targetexhaust temperature is limited to a positive value.
 13. A method as inclaim 10 wherein the adjusted fuel split schedule further comprises anadjusted first fuel split indicating a portion of fuel flow to a primaryfuel nozzle in the combustor and an adjusted second fuel spit indicatinga portion of the fuel flow to be mixed with secondary air entering thecombustor.
 14. A method as in claim 10 wherein the base fuel splitschedule is determined for a base load condition at which the gasturbine is operating at full rated power.
 15. A method as in claim 10wherein steps (b) through (g) are performed in connection with anitrogen oxide (NOx) leveling algorithm.
 16. A method as in claim 10wherein the application of the difference further comprises: deriving anadjusted projected NOx level from the temperature difference; generatinga Q factor as a ratio of a NOx emission baseline and the adjustedprojected NOx level; applying the Q factor to the base fuel splitschedule to generate the adjusted fuel split schedule.
 17. A method asin claim 16 further comprising determining the adjusted NOx level from acorrected value of the temperature difference.
 18. A method forscheduling a fuel split for a combustor of a gas turbine comprising: a.determining a target exhaust temperature corresponding to a desired NOxemission level from the gas turbine at a reference fuel split schedule,wherein the target exhaust temperature is based on at least oneparameter of a group of parameters consisting of specific humidity,compressor inlet pressure loss and turbine exhaust back pressure; b.determining an exhaust temperature error based on a temperaturedifference between a scheduled exhaust temperature and the targetexhaust temperature; c. converting the exhaust temperature error to aprojected NOx level error at the reference fuel schedule, and d.converting the projected NOx level error to an adjustment to thereference fuel schedule.
 19. A method as in claim 18 wherein theadjusted reference fuel split schedule further comprises an adjustedfirst fuel split indicating a portion of fuel flow to a primary fuelnozzle in the combustor and an adjusted second fuel spit indicating aportion of the fuel flow to be mixed with secondary air entering thecombustor.
 20. A method as in claim 18 wherein the reference fuel splitschedule is determined for a base load condition at which the gasturbine is operating at full rated power.
 21. A method as in claim 18wherein steps (b) through (d) are performed in connection with anitrogen oxide (NOx) leveling algorithm.
 22. A method as in claim 18wherein the conversion of the temperature difference further comprises:deriving an adjusted Q factor as a ratio of a NOx base request and anadjusted NOx level derived from the temperature difference; applying theadjusted Q factor to the reference fuel split schedule to generate theadjustment to the reference fuel schedule.